U.S. patent number 4,769,599 [Application Number 06/835,545] was granted by the patent office on 1988-09-06 for magnetometer with magnetostrictive member of stress variable magnetic permeability.
This patent grant is currently assigned to Geo-Centers, Inc.. Invention is credited to Marc D. Mermelstein.
United States Patent |
4,769,599 |
Mermelstein |
September 6, 1988 |
**Please see images for:
( Certificate of Correction ) ** |
Magnetometer with magnetostrictive member of stress variable
magnetic permeability
Abstract
A highly sensitive magnetometer employs a magnetostrictive
amorphous metal core in which a piezoelectric driver induces stress
variations. In the presence of an external magnetic field, the
magnetization of the magnetostrictive core varies in relation to
the induced stress. The amplitude of the variations in
magnetization is proportional to the strength of the external field
and is sensed by a pick-up winding disposed around the
magnetostrictive core. The dynamic range of the device is improved
by employing a bucking field winding around the core to null out
the magnetization of the core. In contrast to the inherent
non-linear characteristic of the conventional fluxgate
magnetometer, the invention inherently provides a linear response
to magnetic field strength.
Inventors: |
Mermelstein; Marc D. (Chevy
Chase, MD) |
Assignee: |
Geo-Centers, Inc. (Newton
Centre, MA)
|
Family
ID: |
25269789 |
Appl.
No.: |
06/835,545 |
Filed: |
March 3, 1986 |
Current U.S.
Class: |
324/244;
310/323.01 |
Current CPC
Class: |
G01R
33/02 (20130101) |
Current International
Class: |
G01R
33/02 (20060101); G01R 033/02 (); H01L 041/04 ();
H02N 002/00 () |
Field of
Search: |
;324/209,244 ;335/3,215
;365/157 ;360/113 ;310/323,328,321 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Proposal for Detection of Magnetic Fields Through Magnetostrictive
Perturbation of Optical Fibers," Amnon Yariv, Harry V. Winsor,
Optics Letters, vol. 5, No. 3, Mar. 1980, pp. 87-89. .
"Metallic-Glass Fiber Optic Phase Modulators," Frank R. Trowbridge,
Ronald L. Phillips, Optics Letters, vol. 6, No. 12, Dec. 1981, pp.
636-638..
|
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Snow; Walter E.
Attorney, Agent or Firm: Wolf, Greenfield & Sacks
Government Interests
U.S. GOVERNMENT RIGHTS IN THE INVENTION
This invention was made by an employee of Geo-Centers, Inc., who
was in the performance of work under Naval Research Laboratory's
contract N00014-84-C-2399 with Geo-Centers. The United States of
America has certain rights in the invention arising out of that
contract, including a nonexclusive, nontransferable, irrevocable,
paid-up license to practice the invention or have it practiced for
or on behalf of the United States throughout the world.
Claims
I claim:
1. A magnetometer for detecting an applied magnetic field
comprising:
a magnetostrictive element comprising a ribbon of an amorphous
metal, said ribbon having a longitudinal axis extending between
opposing ends;
a piezoelectric driver;
a rigid support;
means for attaching the ribbon at opposing ends to the driver and
support;
means for applying an oscillating voltage to the driver such that
the driver applies an oscillating dither stress to the ribbon which
induces magnetization fluctuations in the ribbon of a magnitude
proportional to the strength of the applied magnetic field;
a pick-up coil surrounding the ribbon along the longitudinal axis
wherein the magnetization fluctuations in the ribbon induce an
electromotive force (EMF) in the pick-up coil; and
means for detecting the EMF in the pick-up coil and referencing the
same to the frequency of the oscillating voltage for producing an
output signal proportional to the applied magnetic field.
2. The device of claim 1, further comprising a nulling circuit for
keeping the ribbon within its linear magnetostrictive range and to
avoid hysteresis effects by keeping the ribbon at essentially zero
magnetic field, said nulling circuit comprising:
a nulling coil surrounding the ribbon along the longitudinal
axis;
means for applying a current to the nulling coil for producing a
nulling magnetic field which opposes and substantially cancels the
applied magnetic field so that the field applied to the ribbon is
substantially zero; and
means for measuring the current applied to the nulling coil to
thereby determine the amplitude of the applied magnetic field.
3. The device of claim 1, wherein the amorphous metal is a metallic
glass.
4. The device of claim 3, wherein the metallic glass is
Metglas.
5. The device of claim 4, wherein the metallic glass is Metglas
2605S2.
6. The magnetometer of claim 1, wherein the driver is made of
PZT.
7. The magnetometer of claim 1, wherein the means for applying the
oscillating voltage provides a voltage at a frequency of about the
longitudinal resonant frequency of the ribbon as attached to the
driver.
8. The device of claim 1, wherein the magnetometer has a
sensitivity of at least about 15 .mu.V/nT.
9. The magnetometer of claim 1, wherein the minimum detectable
magnetic field is no greater than about 15 nT/.sqroot.Hz in the
bandwidth from DC to 1 Hz.
10. The magnetometer of claim 1, further comprising means for
adjusting the equilibrium of the ribbon.
11. The magnetometer of claim 10, wherein the adjusting means
permits adjustment of the position of the driver or support so as
to vary the equilibrium tension of the ribbon.
12. The magnetometer for detecting an applied magnetic field
comprising:
a piezoelectric substrate;
a magnetostrictive element comprising a film of an amorphous metal
applied to a surface of the substrate, said film having a
longitudinal axis;
means for applying an oscillating voltage to the substrate such
that the substrate applies an oscillating dither stress to the film
which induces magnetization fluctuations in the film of a magnitude
proportional to the strength of the applied magnetic field;
a pick-up coil surrounding the substrate and film along the
longitudinal axis wherein the magnetization fluctuations in the
film induce an electromotive force (EMF) in the pick-up coil;
and
means for detecting the EMF in the pick-up coil and referencing the
same to the frequency of the oscillating voltage for producing an
output signal proportional to the applied magnetic field.
13. The magnetometer of claim 12, further comprising a first
electrical insulator positioned between the film and the pick-up
coil.
14. The magnetometer of claim 13, further comprising a second
electrical insulator positioned between the film and substrate.
15. The device of claim 12, further comprising a nulling circuit
for keeping the film within its linear magnetostrictive range and
to avoid hysteresis effects by keeping the film at essentially zero
magnetic field, said nulling circuit comprising:
a nulling coil surrounding the film and substrate along the
longitudinal axis;
means for applying a current to the nulling coil for producing a
nulling magnetic field which opposes and substantially cancels the
applied magnetic field so that the field applied to the film is
substantially zero; and
means for measuring the current applied to the nulling coil to
thereby determine the amplitude of the applied magnetic field.
Description
FIELD OF THE INVENTION
This invention relates in general to apparatus for the detection of
magnetic fields and the measurement of their intensities. More
particularly, the invention pertains to the detection and
measurement of DC and low frequency magnetic fields by a
magnetometer employing a novel arrangement in which the amplitude
of stress-induced magnetic flux variations in a magnetostrictive
element is changed by the magnetic field being measured and the
change is detected through the electric signal induced in a pick-up
coil surrounding the magnetostrictive element. Because the
invention is in many respects comparable to the type of
magnetometer known as the "fluxgate" type, the device of the
invention has been named "the magnetostrictive fluxgate
magnetometer".
BACKGROUND OF THE INVENTION
The conventional fluxgate magnetometer depends for its operation
upon the rapid AC magnetization of a pair of high permeability
cores. Each core carries a primary winding and a secondary winding,
one being disposed inside the other. Upon the flow of AC current
through the primary windings, the cores are magnetized. The
fluctuating magnetization induces currents in the secondary
windings. If a DC external field (i.e. the signal field) is
present, the magnetization of the core is increased by the external
field when the AC magnetic field is in the same direction as the DC
field and is decreased when the AC magnetic field in the opposite
direction. By employing two cores and arranging their AC fields to
be 180.degree. out of phase, the signal obtained from the connected
secondary windings is doubled for a DC external field of constant
intensity.
Fluxgate magnetometers are commonly employed for the detection and
measurement of weak magnetic fields. Because of their sensitivity
fluxgate magnetometers are useful for detecting buried or sunken
objects whose presence affects the local magnetic field. Because of
their directional properties, fluxgate magnetometers have been
employed as magnetic compasses.
A major drawback of conventional fluxgate magnetometers is that
their response to magnetic field strength is non-linear because of
the inherent characteristics of the core material. Consequently,
magnetic field strength signals obtained from the conventional
fluxgate magnetometer require sophisticated processing to assure
the accuracy of measurement.
OBJECTS OF THE INVENTION
The principal object of the invention is to provide an improved
fluxgate type magnetometer for the detection and measurement of DC
and low frequency magnetic fields.
Another object of the invention is to provide a magnetometer having
the sensitivity to detect and measure magnetic fields of lower
strength than those detectable and measureable by conventional
fluxgate magnetometers.
A further object of the invention is to provide a magnetometer
whose signal processing requirement are less complex compared to
the requirements of conventional fluxgate magnetometers because the
invention utilizes a core characteristic that is intrinsically
linear whereas the core characteristic utilized in the conventional
fluxgate magnetometer is intrinsically non-linear.
THE DRAWINGS
FIG. 1 schematically depicts a rudimentary arrangement for a
conventional fluxgate magnetometer.
FIG. 2 shows the B vs H hysteresis loop for magnetic material of
the kind typically employed in the conventional fluxgate
magnetometer.
FIG. 3 schematically depicts the arrangement of a conventional
fluxgate magnetometer.
FIG. 4 shows the scheme of a rudimentary embodiment of the
invention employing a magnetostrictive ribbon core.
FIG. 5 schematically depicts a crude but operative embodiment of
the invention that was actually built and tested.
FIG. 6 is a chart of test data obtained with the FIG. 5
arrangement.
FIG. 7 is a more advanced embodiment of the invention that is
exceptionally rugged because the device is virtually all solid
state.
FIG. 8 is a variation upon the FIG. 7 embodiment which enables a
magnetic field nulling technique to be used to increase the
sensitivity of measurement
THE CONVENTIONAL FLUXGATE MAGNETOMETER
FIG. 1 shows the rudimentary arrangement of a magnetometer of the
conventional fluxgate type. The simple fluxgate magnetometer shown
in FIG. 1, depends for its operation on the AC magnetization of a
core 1 of high magnetic permeability by a signal coil 2 surrounding
the core. Disposed around the signal coil 2 is a pick-up coil 3.
Changes in the magnetic flux .PHI. in the core induce an
electromotive force (emf) .epsilon. in the pick-up coil 3. That
induced signal in the pick-up coil is subsequently processed to
obtain a voltage that is a measure of magnetic field intensity. The
emf .epsilon. is given by Faraday's law of electric induction.
##EQU1## where N is the number of turns of the pick-up coil. The
magnetic flux .phi. is equal to the product of the magnetic
induction B and the cross-sectional area A of the core. Hence, the
emf may be rewritten as ##EQU2## The magnetic field intensity B
inside the core is given by
where
H is the magnetic field strength
M is the magnetization, and
.mu..sub.o is the permeability of free space.
The magnetization M of the conventional fluxgate magnetometer core
is given by
where the constant of proportionality between the field strength H
and the magnetization M is the susceptibility .chi.. Substituting
equation (4) into equation (3) yields the relation between the
magnetic induction B and field strength H which is
where the magnetic permeability .mu. is defined as
In general, .mu.=.mu.(H) is a non-linear function of the field
strength. This is readily seen when plotting the B versus H
hysteresis loop of a typical magnetic core material. FIG. 2 shows
such a plot. In that plot, the permeability .mu. is the slope of
the loop and is a function of the field strength H. Substituting
equation (5) into equation (2) yields ##EQU3## where allowance has
been made for time variations in .mu. and H. Time variations are
introduced to the permeability .mu.(H) by imposing a time dependent
field H.sub.1 (t) in addition to the magnetic strength H.sub.o of
the signal field which is either a DC field or a low frequency AC
field. the total field strength H is
Substituting equation (8) into equation (7) yields ##EQU4## If, as
shown in FIG. 3, a second core 5 with a magnetizing field H.sub.1
that is 180.degree. out of phase with the magnetic field H.sub.1 of
first core 4 is placed within the pick-up coil 6, the emf due to
that second core is given by ##EQU5## The total emf is the sum of
equations (9) and (10) and is given by ##EQU6## Hence the total emf
is due to the flux change resulting from the presence of the low
frequency signal field H.sub.o and the change is magnetic
permeability .mu.(H).
The time dependence of the permeability of the conventional
fluxgate magnetometer is expressed explicitly in the following
equation ##EQU7## That basic fluxgate equation shows that the emf
induced in the pick-up coil 6 as a result of the time varying
permeability is proportional to the low frequency signal field
H.sub.o. It is immediately clear that processing of the signal emf
is greatly complicated by the non-linear behavior of the
d.mu./dH.sub.1 term.
DETAILED DESCRIPTION OF INVENTION EMBODIMENTS
In contrast to the conventional fluxgate magnetometer, the
invention utilizes a core having an altogether different physical
characteristics--i.e. the characteristic of magnetostriction. In
the embodiment of the invention shown in FIG. 4, the core 10 is
constituted of a magnetostrictive element. The core element
preferably is a magnetostrictive material such as Metglas 2605S2
which is an amorphous metal substance made by Metglas Products of
Parsippany, N.J. that is sold in the form of a ribbon. The
magnetization in the magnetostrictive amorphous metal core is given
by
where
.chi..sub.o.sup..sigma. is the magnetic susceptability at constant
stress,
d.sup.H is the piezomagnetic modulus at constant field
strength.
The two coefficients in the above constitutive relation may be
estimated from the dipole rotation model for the magnetostrictive
amorphous metal ribbon core. They are found to be ##EQU8## where
M.sub.D is the length of the domain magnetization vector,
H.sub.A is is the anisotropy field,
.lambda..sub.s is the magnetostrictive constant, and
H.sub.o is the low frequency signal field.
Application of a time dependent stress .sigma.(t) to the
magnetostrictive ribbon core 10 by a piezoelectric driver 11 allows
the development of a time dependent magnetization whose amplitude
is proportional to the signal magnetic field H.sub.o. The magnetic
induction B is found from equations (3) and (13) to be
where
Having determined the magnetic induction B, the emf may be computed
from equation (2). It is found to be ##EQU9## Hence, the emf
induced in the pick-up 12 is proportional to the signal field
H.sub.o.
A comparison of equation (16) with equation (12) facilitates a
comparison of the magnetostrictive technique vis a vis the
conventional fluxgate technique. Firstly, the time dependent
saturating field strength (dH.sub.1 /dt) in equation (12) is
replaced by the time dependent stress field (d.sigma./dt) in
equation (16). More importantly, the non-linear field dependent
term H.sub.o (d.mu./dH.sub.1) in equation (12) is replaced by the
simple linear term 3.lambda..sub.s H.sub.o /H.sub.A.sup.2 in
equation (16). The importance of this distinction is discussed
herein in a subsequent passage.
In the rudimentary embodiment of the invention shown in FIG. 4, the
core 10 is a magnetostrictive amorphous metal ribbon which is
secured at one end to a rigid support 13. At its other end the
ribbon 10 is attached to a piezoelectric driver 11 extending from a
second support 14. A sinusoidally varying stress is applied to the
taut ribbon by the piezoelectric driver upon which a varying
voltage is impressed. The piezoelectric driver generates an
internal stress field .sigma.(t) in the magnetostrictive ribbon.
Preferably, the ribbon is driven at its resonant frequency
.omega..sub.o corresponding to a sonic wave traveling longitudinal
along the ribbon.
Examination of the domain constitutive equation (14) shows that a
longitudinal magnetization will develop in the core as a result of
the stress .sigma. only in the presence of a non-zero low frequency
magnetic field H.sub.o (i.e. the signal field) inasmuch as the
piezomagnetic modulus is proportional to H.sub.o. Assuming a
sinusoidal dependence to the stress field
where .sigma..sub.o is the stress amplitude, the induced emf is
found from equations (2) and (14) to be ##EQU10## The amplitude
.epsilon..sub.o of the emf is ##EQU11## and can be readily detected
using a lock-in amplifier (i.e. using phase sensitive detection
techniques).
The sensitivity .eta. of the magnetostrictive fluxgate magnetometer
is defined as the change is induced emf per change in the strength
of the signal magnetic field. ##EQU12## Assuming that the applied
stress field has an amplitude .sigma..sub.o equal to 10% of the
critical stress, ##EQU13## that the pick up coil has 1000 turns,
that the ribbon has a width of 1 cm. and a thickness of 30 .mu.m,
that the magnetostriction constant .lambda..sub.s is 30 ppm, that
the anisotropy field is 80 A/m and that the operating frequency is
10 kHz, the sensitivity is found to be
Hence, a signal magnetic field of 1.0 nT will induce an emf of 15
.mu.V which is readily detectable using conventional synchronous
detection techniques.
FIG. 5 shows the scheme of a crude but operable embodiment of the
invention that was actually constructed and tested. In that
embodiment, the core 15 is a 2".times.1/2" magnetostrictive
amorphous metal ribbon (Metglas 2605S2) which was cut from a 2"
wide ribbon roll. One end of the 2" long ribbon is secured to a
stationary rigid support 16. The other end of the ribbon is
cemented to a 30 mm.times.13 mmm.times.0.7 mm piezoelectric ceramic
plate 17 having thereon spaced electrodes. The piezoelectric plate
is made of PZT5A material. The magnetostrictive ribbon is secured
by an epoxy adhesive to one 13 mm edge of the piezoelectric plate
with a glass slip 18 interposed between the ribbon and the plate to
electrically insulate the ribbon from the electroded surface of the
piezoelectric plate. The opposite 13 mm edge of the piezoelectric
plate is attached to an insulator block 19 mounted on a
micropositioner 20. Adjustment of the micropositioner enables the
amount of tension applied to the ribbon to be changed. Because the
magnetoelastic response of the ribbon is dependent upon the
equilibrium tension, the performance of the ribbon can be optimized
by adjusting the micropositioner to provide the requisite tautness
of the ribbon.
The ribbon is surrounded by two coils 21 and 22, one within the
other. The coil 20 is a pick-up coil having 100 turns of copper
wire. The coil 22 is a magnetic field producing device having about
20 turns of 0.35 mm diameter copper wire. A variable DC power
source 23 supplies current to the coil 22 and produces a DC
magnetic field which in MKS units is given by
where
N=2.9.times.10.sup.3 m.sup.-1, and
I is the current in amperes.
An oscillator 24 is connected to the piezoelectric plate and drives
it at 100 KHz with an AC voltage of about 0.5 volt rms. The
piezoelectric response of the plate stresses the magnetostrictive
ribbon which, in the presence of the DC field established by coil
22, generates an emf in pick-up coil 21. That emf is detected by
the lock-in amplifier 25 which is referenced to the piezoelectric
ceramic plate 17 and monitored by a chart recorder 26 and an
oscilloscope 27.
FIG. 6 is a chart of the magnetic field sensing data measured with
the FIG. 5 arrangement. The data was taken in the following manner:
the stress dither was applied to the ribbon by the piezoelectric
driver and the oscillating emf was monitored by the oscilloscope.
The DC bias field produced by coil 22 was adjusted to minimize the
observed signal. This small stress-induced signal was
simultaneously monitored by the lock-in amplifier 25 and the chart
recorder 26. The current in the coil 22 was then changed by .+-.0.1
mA increments which corresponds to 360 nT steps in the DC magnetic
field. The lock-in output reflects this change in the DC field. the
minimum detectable magnetic field is taken to be equal to the
approximate rms noise level of the lock-in output, i.e. unity
signal to noise ratio. It is found to be approximately 15
nT/.sqroot.Hz in the bandwidth from DC to 1 Hz.
A more advanced embodiment of the invention is schematically
depicted in FIG. 7. In that FIG. 7 embodiment, a film 30 of the
magnetostrictive amorphous metal is deposited by sputtering or some
other suitable deposition process upon the surface of a substrate
31 of a piezoelectric material such as quartz. A voltage from a
source 32 is applied to the piezoelectric substrate 31 in a manner
imposing a sinusoidally varying stress upon the magnetostrictive
film 30. That varying stress, in turn, causes a corresponding
variation in the magnetization of the film 30 in the presence of a
DC magnetic field which is schematically represented in FIG. 7 by
the H arrow.
Surrounding substrate 31 is a pick-up winding 33 which senses the
variations in the magnetization of the film 30 by induction and
provides an emf signal at output terminals 34. The magnetostrictive
film 30 is covered by a thin coat 35 of an electrically insulative
material deposited over the magnetostrictive film to insulate the
pick-up winding 33 from the film 30. When necessary a thin
insulator 36 may be interposed between the film 30 and the
piezoelectric substrate 31, as indicated in the FIG. 8 embodiment.
Major advantages of the FIGS. 7 and 8 embodiments are (1) the
magnetometer can be exceptionally rugged because the device is
virtually all solid state and (2) the magnetometer can be highly
miniaturized because the component of greatest bulk is the
piezoelectric substrate.
The dynamic range of the invention embodiments can be improved by
using a "nulling" technique. When using that technique, the
magnetic field H is nulled out by an opposing magnetic field
established, as shown in FIG. 8, by a "bucking field" winding 37
disposed around the substrate 31. When the magnetization of film 30
is no longer detectable by the pick-up winding 33, the current
applied at terminals 38 of the "bucking field" winding 37 is then a
measure of the intensity of the H magnetic field.
Inasmuch as the invention can be embodied in various forms, it is
not intended that the scope of the invention be limited only to the
embodiments here described. It is intended rather, that the scope
of the invention be construed in accordance with the appended
claims, having due regard for obvious changes that do not depart
from the essentials of the invention.
* * * * *